Processes are known for recovering uranium values from "wet process" phosphoric acid by concentration and purification. The first cycle generally involves extracting uranium from the phosphoric acid to an organic phase generally containing organic extractants such as a combination of a dialkylphosphoric acid and a trialkylphosphine oxide. The second cycle generally involves stripping the uranium from the organic solution with an aqueous phosphoric acid and reextracting uranium into an organic solution containing extractants. This second organic solution is typically washed to remove excess phosphoric acid and stripped with an ammonium carbonate solution to form insoluble ammonium uranyl tricarbonate (AUT). AUT can then be filtered, washed and calcined to yield a uranium oxide product.
In some such processes, the uranium is oxidized to a hexavalent state before each extraction and reduced to a tetravalent state before the first stripping step, since the hexavalent uranium is preferentially dissolved in the organic solution and the tetravalent uranium is preferentially dissolved in the aqueous solution. In some processes, however, the extractant is chosen such that the organic solution preferentially absorbs tetravalent uranium such that uranium is extracted into the organic phase in a tetravalent state and stripped from the organic solution in a hexavalent state.
Such processes suffer from the disadvantages involved in oxidation and reduction of uranium between successive steps. In addition, the preferential solubility of uranium ions in the desired phase is generally not sufficient to effect a relatively complete transfer in the desired direction.